1. Field of the invention
The present invention relates to an apparatus for controlling a self-refresh period in a memory device, and more particularly to an apparatus for controlling a self-refresh period in a memory device which can normally perform a self-refresh operation that is indispensable to the operation of a volatile memory device using a refresh period control even if the inner temperature of the memory is changed.
2. Description of the Prior Art
As is well known, a volatile memory device has the drawback in that it cannot maintain data stored in memory cells over a predetermined time due to leakage current components that the memory cells have.
In order to compensate for such limitations, a system for the memory device performs a refresh operation for restoring data at predetermined intervals. The refresh operation is classified into an auto-refresh operation that is performed during a normal operation of the memory device and a self-refresh operation that is performed in a state that the memory device performs the minimum operation in order to reduce the power consumption as the system does not operate for a long time. In a self-refresh mode, the refresh operation is performed in order to accurately maintain the stored data.
The refresh operation is basically the same as a raw-active operation and a precharge operation that are normal operations. That is, the data stored in the memory cells is amplified by a sense amplifier, and then the data is stored again in the memory cells.
Meanwhile, in the case of the self-refresh operation, the refresh operation should be performed at predetermined intervals without any command from an outside of the memory device, and thus the self-refresh operation is independently performed inside the memory chip.
That is, even if a row-active command is not applied from an outside, a row-active operation is performed, and then a precharge operation is performed.
Hereinafter, the self-refresh operation will be explained with reference to the accompanying drawings.
FIG. 1 is a view illustrating a conventional process of generating a refresh signal when a memory device enters into a self-refresh mode. FIG. 1 shows the process of internally generating a refresh signal srefreq that is a signal for operating the memory device in the same manner as the case that an active command is applied in the self-refresh mode.
As illustrated in FIG. 1, if a self-refresh command is input from the outside, a self-refresh command signal self_refresh obtained by combining signals produced from an internal command decoder (not illustrated) operates an oscillator 110 that operates for a period of t0, and a frequency doubler 120 receives a signal having the period of t0 from the oscillator 110, and generates pulse signals having predetermined periods of 2t0, 4t0, 8t0 and 16t0.
A frequency selection generator 130 is used to produce the refresh signal srefreq of FIG. 1, that is, the signal suitable to be used in the memory device by selecting a proper frequency signal among period signals having passed through the above-described process. A row control unit 140 receives this signal, and outputs a row-active signal for driving a corresponding word line by banks. Additionally, an address control unit 150 receives the signal, and outputs an address signal for operating word lines in an accurate operation order. For reference, a bank control unit 160 controls a plurality of banks included in a core unit 170.
FIG. 2a is a circuit diagram of a frequency selection generator 130 of FIG. 1. Referring to FIG. 2a, it is assumed that a pulse signal 4t0 having a period of 4t0 and a pulse signal 8t0 having a period of 8t0 are received from a frequency doubler 120.
As shown in FIG. 2a, the frequency selection generator is provided with a fuse selector 210 and a generation unit 220, and the generation unit 220 is provided with a selection unit 221, a pulse generation unit 222 and an output unit 223.
A pulse signal having a period suitable to be used in the memory device is selected between two pulse signals 4t0 and 8t0 generated from the frequency doubler 120 in accordance with frequency selection signals Select1 and Select2 generated from the fuse selector 210. One pulse signal selected as above is output as the refresh signal srefreq through the generation unit 220.
FIG. 2b is a waveform diagram explaining the operation of the frequency selection generator illustrated in FIG. 2a. 
As illustrated in FIG. 2b, the frequency selection generator selects the pulse signal 8t0 having a period of 8t0 between the two pulse signals 4t0 and 8t0 received from the frequency doubler 120, and outputs the selected pulse signal as the refresh signal srefreshq. That is, if the self-refresh command signal self_refresh is changed to a high level when the memory device enters into the self-refresh mode, the refresh signal srefreq becomes a pulse signal having a period of 8t0.
As is already known, due to the structural limits of the volatile memory cells, the refresh operation is compulsory, and particularly, even if the memory device enters into the self-refresh mode, the refresh operation should be performed in the same manner.
In a normal operation of the memory device, heat is generated due to the high-speed operation of the memory device, and the inner temperature of the memory device is increased. If the memory device enters into the self-refresh mode in such a high-temperature state, the leakage current of the memory cell itself is increased, and the data is consumed as the leakage current to cause a data loss. In order to prevent such a data loss, the data should be restored by shortening the refresh period.
Meanwhile, as a predetermined time elapses, the inner temperature of the memory device is lowered, and thus the leakage current of the memory cell itself is reduced. In this case, the power consumption should be reduced by reducing the current through the lengthening of the refresh period.
However, according to the conventional apparatus for controlling a self-refresh period in a memory device as described above, the frequency selection generator (See FIG. 2) outputs the refresh signal srefreq having the period of one pulse signal selected through the output signals Select1 and Select2 of the fuse selector 210 when the memory device enters into the self-refresh mode. The refresh operation of the memory device is performed by the refresh signal srefreq at predetermined intervals. As a result, if the refresh period is lengthened in the event that the memory device is in a high-temperature state, a data loss may occur. Meanwhile, if the refresh period is excessively shortened in a state that the inner temperature of the memory device is lowered as the predetermined time elapses, it results in the excessive use of the current in the self-refresh mode to cause great power consumption.